Hmm, At the moment this would be a little tricky. The best way I can think of it would be to add a $has_one to your Page class, something like:
$has_one = array(
'CustomMenuLink' = 'CustomMenu'
);
Then you would add a Dropdown field (or something like that) to the CMS and have a custom method on your page controller, something like:
public function loadMenu() {
return DataObject::get_by_id('CustomMenu',$this->CustomMenuLink()->ID);
}
Then you could add this in your template:
<ul>
<% control loadMenu %>
<li>$Title</li>
<% end_control %>
</ul>
Just a couple of disclaimers:
1. This isn't tested, you will probably have to tweek it to get it working.
2. This should work in 2.4 with a little modification, but I am not sure about 2.3 (or lower).
Hope that helps,
Mo